The present invention relates to photometric devices for determining the exposure interval for a scene to be photographed and, more particularly, to photometric devices which determine the duration of the exposure interval as a function of the median value of the light reflected from the scene.
Typical photometric devices for measuring the light reflected from a scene to be photographed include a photo-responsive cell that produces an electrical output in response to the average or mean value of the illuminance received from the scene. These types of photometric devices generally produce accurate exposure interval determinations when the distribution in illuminance from the different portions of the scene is relatively narrow with the accuracy of the exposure interval determination diminishing as the illuminance distribution increases. For example, in photographing a generally dark evening scene by available light, the accuracy of the exposure interval determination made by the aforementioned averaging type photocells is generally degraded by the presence of a bright point-source of illumination that causes the exposure interval determination for the scene as a whole to be shorter than that desired from an artistic standpoint.
More sophisticated photometers have been developed in an effort to obtain more accurate exposure interval determinations under widely varying scene illumination. These photometers have included multi-cell photo-responsive arrays of various arrangements and other arrangements in which a photocell is positioned to receive the light reflected from the probable subject of primary interest in the central portion of the scene to be photographed and another photocell positioned to receive light from the probable subjects of secondary interest in the peripheral portions of the scene. Electronic circuitry is provided to select one or the other of the photocells or correct the exposure determination of one of the photocells as a function of the output of the other of the cells in an effort to provide more accurate exposure interval determinations. While the multi-cell photometers generally achieve their intended goal, the exposure interval determination is characteristically based upon and is a function of the average light received from the scene.
In view of the above, the present invention provides a photometric device for providing an exposure interval determination for a scene to be photographed as a function of the median value of the light reflected from the scene. A photo-responsive array is provided having a plurality of photocells that are each adapted to receive light reflected from selected portions of the scene to be photographed with each photocell providing an electrical output that is responsive to the light incident thereon. The output of each photo-responsive cell is time integrated with the integrated result for each photocell compared to a predetermined exposure value based upon the exposure parameters of the film (e.g. film speed and aperture opening). The exposure interval is terminated when a predetermined number of time-integrated results, namely, a majority, meets or exceeds the desired exposure parameters for the film.
A photometer in accordance with the preferred embodiment includes a photo-responsive array having a plurality of photocells including a photocell for receiving the light reflected from the probable subject of primary interest in the central portion of the scene and photocells for receiving the light reflected from the subjects of secondary interest in, respectively, the background portion, the foreground portion, and the lateral side portions of the scene to be photographed with each cell providing a photo-current output responsive to the light incident thereon. The output of each photo-responsive cell is time-integrated by a combined integrator/logic device which provides a binary zero output for a time period inversely proportional to the illuminance incident on its respective photocell. When the time-integrated result for each photocell meets or exceeds an exposure parameter value that indicates proper exposure for the film exposure parameters, the combined integrator/logic device for that photocell provides a binary one output. A majority gate logic operator coupled to the integrator/logic devices terminates the exposure interval when a median number or majority of the logic device outputs are binary one.
Preferably, the response of the photo array is adjusted to center-weight the array in favor of the central photocell to enhance the probability of proper exposure of the subject of primary interest in the central portion of the scene.
The principal object of the present invention is to provide a photometric device that advantageously determines an exposure interval that is based upon and a function of the median value of the light reflected from the scene to be photographed so that the exposure interval determination is less adversely affected by scene illuminances having distributions far above or far below the average scene illuminance. Other objects of the present invention include providing a photometric device for cameras in which the exposure interval determination is made prior to film exposure, for cameras having time-integrating exposure systems in which the exposure interval determination is made during the exposure interval, and for stand-alone photometers or exposure meters.